Donald Trump has not released his tax returns, making him the only presidential candidate in modern history to refuse to do so.

He has repeatedly said he can't release them because he's under audit by the Internal Revenue Service, something the government agency has said shouldn't bar him from making his tax returns public.

"As far as my return, I want to file it, except for many years, I've been audited every year," he said during a Republican primary debate in February, according to a transcript by the New York Times. "Twelve years or something like that. Every year they audit me, audit me, audit me.

The IRS released a statement to reporters earlier this year saying there are no restrictions on releasing tax information while being audited.

Donald Trump's tax returns have become a major debate topic.

During Monday night's presidential debate against Hillary Clinton, Trump repeated the claim he can't release his taxes until the audit is over. "I told you, I will release them as soon as the audit," he said. "Look, I've been under audit almost for 15 years."

He even criticized Mitt Romney in 2012 for not releasing his taxes.

Earlier this month, Donald Trump Jr. said his father couldn't release his taxes because it would detract from his message. "Because he's got a 12,000-page tax return that would create ... financial auditors out of every person in the country asking questions that would distract from [his father's] main message," he told the Pittsburgh Tribune-Review earlier this month when asked why his father hasn't released his tax records.

At the debate, Trump said he would release his taxes if Hillary Clinton "releases her 33,000 e-mails that have been deleted." Then he seemed to say he doesn't even pay taxes.

Clinton: Maybe he doesn't want the American people, all of you watching tonight, to know that he's paid nothing in federal taxes, because the only years that anybody's ever seen were a couple of years when he had to turn them over to state authorities when he was trying to get a casino license, and they showed he didn't pay any federal income tax.